This invention relates generally to discharge electrodes for electrostatic precipitators, and more particularly to a discharge electrode structure disposed within a tubular collector electrode and adapted to maintain a uniform high-intensity electrostatic field within the tubular electrode.
Electrostatic precipitators function to separate contaminating particles or droplets of a semi-solid or solid nature from a gaseous stream. Such precipitators are especially helpful in removing finer particles (less than 40 .mu.m). In one known form of electrostatic precipitator, the gases to be purified, such as those issuing from an incinerator, are conveyed through a collector tube where they are subjected to an electrostatic field which causes the particles to migrate toward the inner wall of the collector tube, thereby separating the particles from the gas flowing through the tube. With continued operation of a dry precipitator, the particles accumulate on the wall of the collector tube and it becomes necessary, therefore, at fairly frequent intervals, to shut down the precipitator in order to permit removal of the agglomerated particles.
With a wet wall precipitator of the type disclosed, for example, in the deSeversky U.S. Pat. No. 3,716,966, a uniform film of downwardly flowing water is formed on the inner wall of the collector tube, the film serving to continuously wash away the contaminants, thereby obviating the need to interrupt the precipitator operation. The present invention is applicable to a dry or wet type of electrostatic precipitator in which the discharge electrode structure is coaxially supported within a tubular collector electrode.
In a coaxial electrostatic precipitator of the dry or wet type, unipolar ions are produced by a discharge electrode, the ions migrating across the gap between this electrode and the tubular collector under the influence of an electric field established therebetween. In so migrating, the ions attach themselves to the aerosol particles moving with the gas passing between the electrodes, the charged particles being attracted to the collector.
In one elementary form of coaxial electrostatic precipitator in widespread use, the discharge electrode is a wire coaxially supported within a tubular collector electrode. This wire has a much smaller radius of curvature than the tubular collector, the air gap or inter-electrode space between these electrodes being very large compared to the radius of the wire. When, therefore, a voltage is impressed across these electrodes and the potential difference therebetween is raised, a point is reached where the air near the more sharply-curved discharge electrode breaks down, but only to an extent producing a corona discharge.
The electric field varies inversely with the radius of the wire. For a given air gap dimension, the level of voltage needed to produce a corona discharge is below that necessary to completely break down the dielectric of air to produce a spark discharge across the gap. Since an understanding of this distinction is vital to the invention, the behavior of corona and spark discharges will be further analyzed.
A corona discharge is a highly active glow region surrounding a discharge electrode. In the above described elementary form of precipitator, this electrode is constituted by a wire, the glow region extending a short distance beyond the wire. Assuming that the wire is negatively charged, the free electrons in the gas in the region of the intense electric field surrounding the wire gain energy from this field to produce positive ions and other electrons by collision. In turn, these new electrons are accelerated and produce further ionization.
This cumulative process results in an electron avalanche in which the positive ions are accelerated toward and bombard the negatively-charged wire. As a consequence of such ionic bombardment, secondary electrons are ejected from the wire surface which act to maintain the discharge. Moreover, high-frequency radiation originating from excited gas molecules lying within the corona region contribute to the supply of secondary electrons.
The electrons emitted from the negatively-charged wire or discharge electrode are drawn toward the positively-charged collector electrode. As these electrons advance into the weaker field away from the wire, they tend to form negative ions by attaching themselves to neutral oxygen molecules. These negative ions create a dense unipolar cloud that occupies most of the gap between the electrodes and constitutes the only current in the entire space outside the corona glow region. This space charge functions to retard the further emission of negative charge from the corona region and in this way restricts the ionizing field adjacent the wire, thereby stabilizing the discharge.
When, however, the voltage applied to the ionizing electrode is further elevated to a level exceeding the point at which a corona discharge is maintained in a stable condition, the air dielectric then completely breaks down, as a result of which the air in the gap is rendered relatively conductive to sustain a spark discharge which is accommpanied by a heavy current flow.
An electrostatic precipitator attains its highest operating efficiency under optimum ionization conditions when the voltage applied to the discharge electrodes approaches the point of transition between an incomplete breakdown or corona discharge producing a copious supply of ions and complete air dielectric breakdown or spark discharge which effectively short circuits the precipitator and renders it inoperative.
But in practice one must be careful to apply a voltage to the ionizing or discharge electrode of a precipitator which is well below the level at which complete air breakdown is experienced, for the air breakdown characteristics of air in a precipitator varies with the nature and concentration of the pollutants therein as well as barometric pressure conditions. Moreover, the breakdown of the dielectric of air produces chemical reactions which constitute a serious health hazard; for this breakdown gives rise to toxic ozone and harmful oxides of nitrogen. But quite apart from this health hazard is the fact that ozone is highly reactive with electrical insulation and other structures and therefore has a destructive effect on the associated equipment.
When using a single ionizing wire as the discharge electrode, two problems are encountered whose possible solutions work at cross purposes. First, there is the problem of maintaining a uniform voltage gradient from the ionizing wire which extends along the axis of the collector tube to the inner wall of the tube. Second is the problem of maintaining the power supply voltage at a reasonable level in order to produce the desired high voltage gradient.
With a single-fine-gauge discharge wire extending coaxially within the collector tube, the voltage gradient about this wire to the suface of the tube (usually grounded) is uniform over a 360.degree. polar angle from the center of the wire normal to its axis. We shall now, by way of example, assume a collector tube having an 8-inch diameter, a discharge wire having an 8-mil diameter and an excitation voltage of 30 kilovolts applied between the wire and the collector tube. The resultant voltage gradient between the wire and the collector tube is then approximately 7500 volts per inch. Because the discharge wire is of small diameter and is at a high voltage, it will readily ionize small particles suspended in a contaminated gas passing through the tube. As a result, the ionized particles will migrate toward the wall of the collector tube, this migration being induced by the intense voltage gradient.
If now one wishes to scale up a precipitator structure of this type so that it is then capable of handling a greater volume of contaminated gas, this can only be done by enlarging the diameter of the collector tube to, say, 16 inches. This doubles the distance between the center discharge electrode and the grounded collector tube and therefore reduces the voltage gradient between the discharge electrode wire and the grounded collector tube to about 3.75 KV per inch, or to one half of the previously effective value. As a consequence, fewer of the particles that are ionized are driven to the wall of the collector tube. This is particularly true of larger particles that have a high mechanical inertia and also of extremely small particles that do not readily accept a charge.
In order to raise the efficiency of the enlarged (16") precipitator to the level attained by the smaller (8") precipitator, the obvious step is to increase the voltage from 30 KV to 60 KV and thereby establish the same voltage gradient. The drawback to this obvious approach is not only that it entails a far more costly power supply, but now that one has doubled the voltage to maintain the same voltage gradient, the likelihood of an air breakdown that would short circuit the precipitator is greatly augmented.
Alternatively, rather than step up the power supply voltage, one could, upon enlarging the diameter of the collector tube, also expand the diameter of the coaxial discharge electrode wire. Thus when using a 16-inch diameter collector tube one could at the same time change the discharge electrode wire from an 8-mil diameter to one having an 8-inch diameter. But this solution is not effective; for a large diameter wire has poor air-ionizing characteristics.
Another approach to the problem is that disclosed in the above-identified deSeversky patent in which the discharge electrode is defined by a circular array or cage of wires. Thus in the case of a 16-inch collector tube, one may place an array of small gauge ionizing wires in a circle whose periphery is spaced four inches from the inner wall of the collector tube. The difficulty with this approach is that even though it enhances the voltage gradient, the wires themselves, because of their proximity to each other, create a non-uniform gradient in the air, as a consequence of which many particles in the gas passing through the tube are not adequately ionized and are therefore not collected.